It is now well established that work-hardening plasticity models [1–۳] can describe accurately the plastic response of a shallow foundation under planar static combined loads (V, M, H). These models rely on the assumption that, following a given penetration, w, a yield surface develops in the load space. For a foundation of diameter D or breadth B (Fig. 1a) resting on a frictional material, the yield surface can be described by the equation [4] H V0h0
۲ þ M=B V0m0
۲ ۲a H V0h0 M=B V0m0 4 V V0 1 V V0  
۲ ¼ ۰ ð۱Þ which is a cigar-shaped envelope, parabolic in sections containing the axis of vertical load (V, M/BH) and elliptical in planes at constant vertical load (H, M/B). Parameters m0 and h0 provide the maximum moment and horizontal dimension of the surface, while parameter a defines the rotation of its elliptical sections, as displayed in Fig. 1b. According to Eq. (1), the yield surface scales with V0, the vertical load mobilised by the plastic component of penetration, while maintaining its shape unvaried. Single gravity and centrifuge experimental campaigns have shown that this framework essentially holds for foundations resting on the surface of homogeneous sandy samples [4–۶]. The effects of the foundation embedment on the yield surface, as mobilised by the foundation penetration, w, or initial depth, d, and typically up to the foundation breadth (or diameter), have been addressed since the end of the nineties, mostly based on experimental observations. For a shallow foundation resting on the surface of a loose sand sample, the maximum horizontal dimension of the normalised yield surface (Eq. (1)) was shown to increase linearly as a function of the sole penetration, w [7]. This trend was also observed on medium dense sand samples [8]. Results of tests on buried foundations on very dense sand showed that the size of the normalised yield surface expands following a linear trend as sole function of the initial foundation depth, d, along both the horizontal and moment dimension, with a similar rate [9]. Experiments on flat plates provided with peripheral skirts on dense sand [10], showed that the size parameters, while increasing with the skirts depth, d, also decreases as a function of V0, reaching a minimum at the maximum allowable vertical load, Vpeak. A significant influence of the effects of the skirts on the yield surface rotation (parameter a) was also observed in the study. An increase in the normalised yield surface size and rotation due to the presence of skirts about the foundation perimeter was also observed in loose sand [11]. These tests also showed that, in presence of very low values of vertical load, the normalised yield surface might extend in the tensile range of vertical load, a tendency also observed more recently on dense sand [12]. Results of centrifuge tests on buried foundations in sand samples of medium density displayed a normalised yield surface function of the foundation initial depth, d, and penetration,